The present invention relates generally to the manufacture of small electric motors, and, more particularly, to the manufacture of motors known as printed motors, in which the motor armature takes the form of a disk fabricated with the use of printed circuit techniques.
The armature, and the armature conductors printed on it, rotate in a flat annular air gap in a plane perpendicular to the shaft of the motor. Many of the advantages of the motor are derived from the fact that the rotating portion contains no iron. In particular, the torque of the motor is generated smoothly without any "cogging" between preferred armature positions. The motor armature also has a very rapid response to changes in terminal voltage, because of its low inertia. Consequently, the printed motor is particularly useful in applications where speed control is critical, such as in servo motors, or as a drive motor for video tape recorders.
Basically, a printed motor comprises a generally cylindrical housing, of relatively short axial length, an array of field magnets arranged in an annular assembly within the housing, a disk-shaped armature, and two flux plates, one of which may be referred to as the inner flux plate and is mounted in the housing in contact with one coplanar face of the magnet assembly, and the other of which may be referred to as the outer flux plate and is spaced apart from the opposite face of the magnet assembly, to form an annular air gap. A magnetic circuit is thus formed by the magnets, the air gap, and the two flux plates, and the armature is mounted for rotation in the air gap. Unlike conventional motors, the printed motor has no separate commutator, but has brushes which bear directly on the printed armature.
A critical dimension in the printed motor is the axial length of the air gap in which the armature rotates. In the past, this dimension has been controlled as well as possible by machining the inside surface of the housing to a desired depth, and ensuring that the dimensions of the inner flux plate and the magnet assembly are accurately controlled, so that the axial length of the resulting air gap is also controlled as accurately as possible. However, control of these several dimensions gives rise to a cumulative error in the gap, especially over relatively long production runs, and the gap lengths may vary significantly from motor to motor. Since the gap length is determinative of the magnetic field strength in the gap, and, therefore, is also determinative of the performance characteristics of the motor, manufacturers must usually underrate their motors to be sure that all of the motors manufactured meet published performance specifications. Otherwise, a motor with an air gap greater than the design value would be unable to generate its specified maximum torque without exceeding a safe armature current level.
It will be appreciated from the foregoing that there is a need for a simple and convenient technique for accurately controlling the gap length of motors of the printed type. The present invention satisfies this need.